My version of your task

Anna'sProgramm.c

//
//  main.c
//  AnnaTest
//
//  Created by Oleg Demchenko on 10/29/14.
//  Copyright (c) 2014 Oleg Demchenko. All rights reserved.
//

#include <stdio.h>
#include<ctype.h>

int main(int argc, const char * argv[]) {
    int x, x1, x2, x3, y, y1, y2, y3;
    
    set_coordinates (1, &x, &y);
    
    set_coordinates (2, &x1, &y1);
    
    set_coordinates (3, &x2, &y2);
    
    set_coordinates (4, &x3, &y3);
    
    if (x == x3 && x1 == x2 && y == y1 && y2 == y3 && y1 != y2 && x1 != x3)
        printf("mozno\n");
    else
        printf("nelze\n");
    
    return 0;
}

int set_coordinates (int pointNumber, int* x, int* y)
{
    printf("Point #%d\n", pointNumber);
    if (get_integer("Enter X\n", &x)) {
        ; // End of file or I/O error (rare)
    }
    if (get_integer("Enter Y\n", &y)) {
        ; // End of file or I/O error (rare)
    }
    
    return 0;
}

int get_integer(const char *prompt, int *i) {
    int Invalid = 0;
    int EndIndex;
    char buffer[100];
    do {
        if (Invalid)
            fputs("Invalid input, try again.\n", stdout);
        Invalid = 1;
        fputs(prompt, stdout);
        if (NULL == fgets(buffer, sizeof(buffer), stdin))
            return 1;
    } while ((1 != sscanf(buffer, "%d %n", i, &EndIndex)) || buffer[EndIndex]);
    return 0;
}